Salmonella enterica serovar Typhimurium is a gram-negative facultative intracellular pathogen. Serovar Typhimurium infections usually result from ingestion of contaminated food or water. The organism generally targets and colonizes the intestinal epithelium of the host and causes gastroenteritis (i.e., salmonellosis). During a Salmonella infection, the growth phase and growth conditions of the organism are important in attachment, invasion, and the regulation of many of the virulence genes. Cells grown under limited oxygen concentrations are more invasive and adhere better to mammalian cells than do aerobically grown or stationary-phase cells. Salmonella invasion genes have been identified and localized. During infection, serovar Typhimurium must adapt to changes in [O2] encountered in the gastrointestinal tract of the host. In Escherichia coli, transitions from aerobic to anaerobic environments or vice versa, involve changes in a large number of genes. However, upon sudden reappearance of oxygen, these cellular processes must be reversed in a precise and orderly fashion to ensure the safe transition to the oxygenated environment. This complex regulatory system has been extensively studied in E. coli, where the DNA-binding protein FNR encoded by fnr, senses changes in [O2] and controls the expression of the different genes either alone or in cooperation with other regulators, e.g., ArcA.